The invention relates to a method for processing sound, in which method a sound field of an ambient sound is measured; a sound field of opposite sign in relation to the sound field of the ambient sound is produced by an actuator; a desired value signal is further applied to an actuator and a sound pressure of the ambient sound is measured with a sensor attached to the actuator, and the difference of the sensor signal and the desired value signal is applied to the actuator, and sound is produced in accordance with the desired value signal, and ambient sound not pertaining to the desired value signal is simultaneously attenuated.
The invention further relates to an apparatus for sound processing, the apparatus comprising an acoustic actuator, a sensor for measuring sound pressure attached thereto, means for applying a desired value signal to the actuator, and means for applying to the actuator the difference of the sound pressure, measured with a sensor, and the desired value signal.
It is known to attenuate sound with such a method that the sound pressure of a sound field and the particle velocity are measured, and on the basis of the measuring results actuators, in other words, means for producing attenuation sound are controlled in order to produce a sound field which is otherwise exactly the same as the original sound field, but of the opposite sign. As a consequence the sound field can be completely suppressed by means of the actuators. The method is applicable to sound attenuation, but production of a desired sound requires separate, complicated solutions.
German Patent Publication 3 025 391 discloses a method and an apparatus, in which sound is attenuated by measuring the sound field and by producing a sound field of opposite sign in relation to the original sound field. The supply of a desired value signal through a sound attenuating actuator is also disclosed. The applications of the solution are somewhat limited, since the measured sound field also comprises sound produced by an actuator.
U.S. Pat. No. 5,018,203 and U.K. Patent Publication Nos. 1 530 814 and 2 160 070 disclose solutions for hearing protectors, in which a sound field is measured and a sound field of opposite sign is produced by a transformer in the hearing protector for attenuating the sound. The supply of an useful signal through a transformer in the hearing protector is also disclosed. By means of hearing protectors, sound entering the ear is attenuated by preventing the penetration of the ambient sound. The applications of the solutions are somewhat limited, and for instance, with these solutions sound cannot be absorbed.
The object of this invention is to provide a method and an apparatus by which it is possible to absorb ambient sound, to produce desired sound, and to produce simultaneously a microphone signal comprising substantially ambient sound only, even though a signal is applied to an actuator for producing sound simultaneously.
The method according to the invention is characterized in that sound is attenuated by absorbing it with an actuator, the effect of a desired value signal on the sound pressure, measured with a sensor, is eliminated.
The apparatus according to the invention is further characterized in that the actuator is arranged to absorb sound, and the apparatus is arranged to eliminate the effect of the desired value signal on the sound pressure, measured with a sensor.
The basic idea of the invention is that a desired value signal is applied to the actuator, which signal the actuator reproduces, and that a sensor measuring sound pressure is attached to the actuator, and that the difference of the sensor signal and the desired value signal is applied as a high gain feedback to the actuator. Then sound not pertaining to the desired value signal is simultaneously absorbed. Further, another basic idea is that the effect of the desired value signal is eliminated from the measured signal, and a microphone signal proportional to a sound not pertaining to the desired value signal is thus produced.
An advantage of the invention is that therewith sound, not pertaining to the desired value signal, can be absorbed in a simple way, and sound in accordance with the desired value sound can be produced simultaneously with the same apparatus, as well as a microphone signal proportional to sound not pertaining to the desired value signal, and all with the same apparatus.